Costa is one of 28 Democrats who voted for Trade Promotion Authority last week — a vote that would give President Barack Obama the power to negotiate a trade agreement with Pacific Rim nations. Unions, which say Obama's plan will cost jobs in the U.S., have frozen campaign contributions to Democrats and threatened to put up candidates in primaries against those who support Obama's trade agenda.

"Costa has voted against labor, he’s voted against the American people and American jobs by voting for TPA and TAA, and we're strongly trying to recruit other folks into the mix to get someone who is friendly to working people and working families," said Rob England, a political coordinator for the Communications Workers of America.

Union leaders in the Golden State declined to comment on who they are in talks with to run against Costa next year. But three Democratic operatives in California said union groups are courting state Assemblyman Henry T. Perea.

The son and namesake of a member of the Fresno Board of Supervisors, Perea is term limited in the state Assembly. Democratic operatives said Perea was already contemplating a primary challenge against Costa, but the trade kerfuffle has heightened union groups' interest in him.

Perea said he has not met with anyone regarding a primary bid, but wouldn't talk further about his plans.

Costa's office did not return a request for comment on Tuesday. But he has been a supporter of Obama's trade initiative and praised it in an interview with CQ Roll Call on June 12.

"I think he made a very convincing argument that the issues that the caucus cares about — the displaced workers and the effects of global trade — the best way to address those negative, adverse impacts is through putting together a trade agreement that speaks to that," Costa said.

Costa has had close calls for re-election in the past, including last cycle, when he won by roughly 1,000 votes against a little-known Republican, but he has never had a primary struggle.

He would start a potential primary with a financial advantage, as he ended March with $620,000 in the bank.

Obama has promised to assist any Democrat who votes for his trade package and draws a primary challenger.

Costa is not the only California Democrat whom unions are threatening. Rep. Ami Bera, a Democrat from the Sacramento-based 7th District, has drawn scorn from union groups over his support of Obama's trade agenda. Unions went up with a six-figure ad buy hitting Bera on supporting Obama's trade package.

But there are no big names floating to challenge Bera. And a primary challenge in the 7th District — one of the most evenly split districts in the country — is an expensive affair. Sacramento is the 20th most expensive media market in the country, according to a Nielsen ranking .

It's much cheaper to run TV ads in Fresno, where Costa's district is located, which makes it less of an investment for union groups. And unlike Bera's seat — which is competitive in a general election — Costa holds a Safe Democratic district, according to the Rothenberg & Gonzales Political Report/Roll Call race ratings . Obama carried it by a 19-point margin in 2012, making it less of a risk of losing an otherwise labor-friendly Democrat.

To be sure, a Supreme Court case on redistricting could completely change California's congressional map — potentially creating more Democratic opportunities in the Central Valley where Costa's district is located. Any possible primary opponents will likely hold out on a decision until it's clear what the district lines will look like.